A statistical characterization of plasmasphere density structure and boundary locations

Abstract

A statistical study of plasmaspheric density profiles and their boundaries is performed, using measurements of core (<50 eV) ions by the retarding ion mass spectrometer (RIMS) on Dynamics Explorer 1. The plasmasphere density profiles are classified into essentially six different categories, indicative of density structure out to the outer boundary of observable cold, isotropic plasma. The most common profiles observed are those which are relatively featureless out to this outer boundary and those which exhibit multiple plateaus and plasmapauses. The multiple plateau profiles occur predominantly in the afternoon and evening sectors, while the “featureless” profiles are most common in the midnight through morning sectors. In the multiple plateau profiles, the average MLT (magnetic local time)‐L locus of the inner “plasmapause” gradients is nearly circular at L = 3–4. Profiles with significant density troughs occur most often in the evening sector, and the troughs are widest in this sector as well. The MLT‐L local time shape of the outer boundary of RIMS‐observed cold ions (termed here low‐energy ion transition, or LEFT) is similar to previously reported average plasmapause shapes, though its bulge region occurs in the afternoon sector. This MLT‐L shape also tends to become more circular with increasing geomagnetic activity. Using simultaneous plasma density measurements from the plasma wave instrument, it is found that the LEIT typically is located between densities of 10 and 100 cm−3, with an average plasma density there of about 60 cm−3.

Development of shoulders and plumes in the frame of the interchange instability mechanism for plasmapause formation

“…This is consistent with whistler motion showing evidence of inward plasma convection in the night sector during substorms, but no outward drift of whistler ducts in this MLT sector. The vestigial plasmapause is still present, so that the cross-L density profile in the post-midnight sector has two separate ''knees'', as sometimes observed by satellites [Horwitz et al, 1990]. Once created, the shoulder subsequently corotates (see Figure 1b) with the unperturbed inner core of the plasmasphere wherein the substorm electric field has not penetrated.…”

Improved analysis of plasmasphere motion using the VLA radio interferometer

“…Notice that the westward events (negative values) still travel with a higher speed than the eastward events (positive values) from zenith > 0 ), disturbed magnetic times (high K 0 p ), and evening hours. They labeled these events ''retrograde'' due to their apparent anticorotational azimuth, and tentatively interpreted them as ''peeled off outerregions of the plasmasphere,'' due to their affinity for high-K p (Horwitz et al, 1990;Chappell, 1974). This study identifies 886 retrograde events, i.e., events in Figure 14 shows the trace velocity distribution of both the MED and retrograde events.…”

Simulation of field-aligned H&lt;sup&gt;+&lt;/sup&gt; and He&lt;sup&gt;+&lt;/sup&gt; dynamics during late-stage plasmasphere refilling

“…Observations (Carpenter, 1966;Horwitz et al, 1990a;Su et al, 2001) show that the differences between these values and more typical numbers -1000 cm −3 at L=3 and 500 cm −3 at L=4 (Corcuff et al, 1972;Horwitz et al, 1990b;Tu et al, 2006;Rasmussen and Schunk, 1990, and references therein) -lie in the dynamic nature of the plasmasphere. That is, these simulations correspond to a prolonged quiet period and, further, do not include a steady-state loss function such as the plasmaspheric wind of Lemaire and Schunk (1992).…”

“…Comparisons of the H + and He + plasmaspheres are of interest because the extreme ultraviolet (EUV) imager on board the IMAGE satellite shows the He + plasmasphere and its structure, which is generally assumed to correspond to that of the higher density H + plasmasphere. However, observations show that He + densities can exceed H + densities during early-stage refilling (Horwitz et al, 1990a;Denton et al, 2002).…”

“…The outward convection and opening of flux tubes, which occurs sunward of the plasmapause (Nishida, 1966), is greatly enhanced by geomagnetic activity and can cause significant global erosion of the plasmasphere (Carpenter, 1966;Song et al, 1988;Lockwood et al, 1990;Chi et al, 2005). It can also cause other observed phenomena, such as multiple plasmapauses in some sectors (Horwitz et al, 1990a;Sandel et al, 2001). Extreme ultraviolet (EUV) images of the He + plasmasphere from the IMAGE satellite (Burch, 2000) show that the plasmapause radius can shrink by approximately one L-shell (from L=4 to L=3) in less than three hours (Goldstein et al, 2003).…”